Application Note

4D Study of Silicon Anode Volumetric Changes
in a Coin Cell Battery using X-ray Microscopy

2 mmAuthors: Dr. Claus Fischer		VARTA Microbattery GmbH		Luke Hunter		Carl Zeiss X-ray Microscopy Inc.

Date:January 2019

Battery capacity is a key area of development that influences our everyday lives. To date, most work has focusedon the electrochemical side. However, as the chemistry and geometry of batteries change, there is more andmore emphasis on understanding microstructural changes. In this work, VARTA Microbattery examines how theZEISS Xradia Versa can be used to evaluate volumetric changes in a silicon anode coin cell, a material which canprovide tremendous capacity improvements but also presents new volumetric challenges versus today’s graphite
anodes.

Introduction

advances in the active materials. These are outlined in Figure 1

Lithium Ion batteries provide energy for every day mobile life.

which, shows some of the key metrics involved with either

Today’s communication, mobile self-monitoring, and entertain-approach, not least of which are performance and safety.

ment choices would not be possible without the extensiveRegarding chemical changes for the active material, silicon as

development of lithium ion batteries over the last 20 years.

an active material for the anode is under strong consideration.

Additionally, areas such as internet of things (IoT), smart cities,

The advantage of silicon would be a large increase in specificand large-scale energy storage are key drivers pushing capacity

capacity. In theory, silicon can provide up to 10X improvement

developments for batteries. Without continued—and potentiallyover graphite. However, a major drawback with silicon is aaccelerated—capacity improvements, these megatrends could

substantial increase in volumetric change during charging

be delayed in reaching their full potential.

and discharging, resulting in expected improvements of only
~1.5X to 3X over graphite—still substantial gains over today’s

The history of commercially available lithium ion batteries

technology. In literature (e.g. Journal of Power Sources 196

started in the 1990s with Sony’s development of the Lithium(2011) 13–24), different possibilities have been discussed toCobalt Oxide (LCO) cell. Over the last 25 years, cell capacity andsolve the volumetric problem as changes during cycling needenergy density has increased stepwise by the implementation

to be considered during the cell design. One possible routeof new materials, better production processes, and higher

might be nano-structuring of active material which can enableefficiency in volume usage. Today, spirally wound 18650 batteries

much better coping of stress than bulk materials. A second

can easily reach energy densities of 700 Wh/l and more.

route would be the use of composite materials, where dramaticvolume changes of silicon can be buffered by a matrix

Paths towards increased cell capacity

material ensuring material grain integrity. In either case,To increase capacity in batteries today, two routes are possible:

understanding the microstructural evolution during charge

geometrical changes for more efficient volume use or chemicaland discharge will be key to optimizing a solution.

Approval Scenario• Mechanical stability• Permeability

Volume usage
Cell housing• Safety• Performance

AnodeCapacity
enhancement• Yield loss
• Self discharge

MaterialsDead space• Safety• Self discharge• Performance• Processability

Cathode• Processability

Figure 1 Pathways for increased battery capacity2Increasing the energy density through greater volume efficiency

High resolution tomography with XRM

is typically done by reducing dead space or reducing non-To observe volumetric change of the battery structure in a

active electrochemical materials, e.g. optimization of separator

single coin cell, 3D tomographic data was collected using athickness, current collector foil thickness, or thickness ofZEISS Xradia Versa. As shown in Figure 3, Versa utilizes micro-housing material. All these changes require advances inscope objectives, in a unique detector design, which decouples

fabrication process technology and are ultimately still

geometric magnification with resolution. This allows muchlimited by the chemistry for maximum charge capacity.

greater flexibility than traditional CT, enabling such capabilities
as Scout-and-Zoom and Resolution at a Distance (RaaD). Please

Volume changes of active materials

see the ZEISS website for additional literature on these concepts

Numerous factors contribute to volumetric changes within

and others XRM related topics.

a battery. Lithium intercalation, electrolyte wetting, and SEI
growth all effect swelling of the active material. In current
graphite based structures, this can result in an overall swellingof 12-15% under normal operation. Although silicon has great
potential for increased charge capacity, the swelling can bein orders of magnitude more than graphite resulting in a very
challenging environment for the engineering of the overall
battery structure.
Materials and methods
The aim of this investigation was to observe the swelling effects
of new anode materials, namely silicon. The underlying question
was: how might cell design be affected by volume changes?
To gain initial insight into this question, an investigation was
carried out on VARTA CoinPower ® CP1254, modified to havesilicon as the active material in the anode. This rechargeable
lithium ion coin cell has a diameter of 12 mm and a height of
5.4 mm. An example of this type of coin cell battery, along withlarger form factor versions, is shown in Figure 2. In the current
generation A3 (graphite anodes) capacity of cell is 60 mAh.

Figure 3: Versa architecture (top) vs. Traditional CT architecture (bottom)

VARTA’s first generation of Si based CP1254 A3 batteries are
expected to have capacities of 75-80 mAh, allowing for ~20%

The coin cell was imaged at different charge states: a) fresh

longer run times between charges, a substantial improvement.

activated cell, directly after electrolyte dosing, before formation;b) after first charge up to 4.2 V voltage level; and c) after firstdischarge down to 3.0 V voltage level. Utilizing Scout-and-Zoom
on ZEISS Xradia Versa, an overview tomography was collected
and then used to identify and navigate to a specific region ofinterest (ROI) for higher resolution imaging. For this study, thesame ROI was imaged at each charge state of the coin cell.
The overview scan was collected at ~5 μm voxel resolution overthe entire sample using the flat panel extension (FPX) while theROI scan was collected at ~2.5 μm voxel resolution utilizing
RaaD and one of the microscope objectives. Figure 4 shows aFigure 2: VARTA CP1254 (and larger siblings) Li-Ion Coin Cell

volumetric rendering of the ROI scan (yellow) within the fulloverview scan (blue). Figure 5 provides a view from a single 2D
virtual cross section of a high resolution scan. The exceptional
contrast and resolution of Versa allows clear visualization of thedifferent elements in the “jelly roll” structure.

3Results and discussion
Figure 6 shows results of analysis between the uncharged state(“0 V”) to fully charged state (“4.2 V”). False color representations clearly show that displacement of the copper foil isa function of position in the jelly roll. The further away fromcenter, the larger the displacement. Knowing that the centerof jelly roll is a mechanical stable winding core, it seems to bequite obvious that swelling will lead to an increase in the jellyroll diameter and sub-sequent stress on the external housing.

Figure 4: Volume rendering of FFOV (Full Field of View) and ROI scans.

Figure 5: Virtual cross section of the ROI scan highlighting the various
components of the battery jelly roll.

Using Volume Graphics VGStudio MAX 3.0, displacement
differences were calculated between the different charge states.
VGStudio is especially suited to this type of analysis with its
proven measurement capability as well as its capacity to handle
large data sets. In this analysis, at times up to four data sets
(20+GB) were loaded at the same time. As demonstrated in
Figure 4, the ROI was positioned and indexed to the overview
scan. After registering the position of the “0 V” (uncharged),“4.2 V” (initial full charge), and “3.0 V” (first discharge) highresolution ROI scan data, the displacement of copper foil

Figure 6: Displacement mapping after initial charge state. Greatest
displacement, as expected, is in the outermost layers and exceeds 100 μm.

between each state was calculated and visually represented.
The copper foil was specifically chosen due to its high contrast

Repeating the calculation from the fully charged state (4.2 V)

which enabled easier tracking of displacement changes for eachto the discharged state (3.0 V), the false color representation

charge state.

reveals a more interesting behavior, shown in Figure 7. The
outer layers do not seem to move at all, whereas the innerlayers start to move away from the center. The expected
contraction of layers, movement of outer layers back towards
the center of jelly roll, does not occur. Instead the inner layers
have continued to displace, presumably after some outer layer
relaxation during the discharge.

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some of the freed area and will expand. The first explanation forthis observation might be the different forces on the particles in
the coating—the inner coating is under compression, whereas
the outer coating is under tension. What this ultimately means
in long term performance and capacity needs to be furtherinvestigated.

Figure 7: Displacement mapping from initial charge to initial discharge. At thispoint, radial displacement occurs more prominently in the inner layers withdisplacements ~1/5 those of the inital displacements (~20 μm).

When examining the thickness of the anode coating layer
by layer, another oddity was observed. In CoinPower ® cells,
a double side coated anode is used, as can be seen in detailpicture Figure 8. During original production these layers arenominally the same thickness. The Inner electrode area isFigure 8: Analysis of anode coatings show that thickness depends on which

facing the center of the battery and the outer electrode area isside of the foil, inner or outer, is being measured. As fabricated, these layers

facing the housing material. Interestingly, these layers do not

are nominally the same thickness.

stay equivalently thick depending on the charge or discharge
state. It seems that during the first charge, the outer electrodeincreases in thickness, while the inner electrode does notchange much. During the first discharge the outer electrode
thickness decreases, but the inner electrode area will harvest

5Summary and future outlook

complete battery system, not only the electrochemistry, but alsoThis study has shown the effectiveness of high resolution

the microstructural and volumetric changes.

x-ray microscopy (XRM) to obtain meaningful and insightful
volumetric information for the evaluation of new materials inFuture work could include similar displacement studies

Li-ion batteries. By utilizing the unique RaaD and Scout-and-throughout the battery life cycle, or effects of nano-structuring

Zoom capabilities of Xradia Versa, VARTA researchers were ableor composites on displacement. Non-destructive investigations

to quantify displacement during initial charging and dischargeof this type could also be expanded to other types of battery

cycles for a coin cell battery which had silicon as the primarygeometries, including pouch cell and larger jelly roll batteries

anode material. The details learned have led VARTA to newsuch as 18650 cells. Regardless of the direction, the amount ofinsights, and new questions, all helping to advance battery

new knowledge gained by such studies could catapult battery

development. This type of study opens the door for VARTA anddevelopment to the next level, thereby advancing a number ofother battery manufacturers to a better understanding of thesocio-economic megatrends.

We would like to acknowledge the contributions of Martin Kronig and Christof Reinhart from Volume Graphics (Heidelberg, Germany)for providing the analysis and visualization.

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Carl Zeiss Microscopy GmbH07745 Jena, Germany
microscopy@zeiss.com
www.zeiss.com/microscopy

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EN_44_013_046 | CZ 01-2019 | Design, scope of delivery and technical progress subject to change without notice. | © Carl Zeiss Microscopy GmbHNot for therapeutic use, treatment or medical diagnostic evidence. Not all products are available in every country. Contact your local ZEISS representative for more information.